Polyanion-type sodium superionic conductor (NASICON) Na₃V₂(PO₄)₂F (NVOPF) is a promising cathode material for sodium ion batteries (SIBs). However, NVOPF shows relatively low specific capacity and poor long-term performance at high rates. Herein, we report a remarkable improvement of NVOPF cathode material by introducing a surface coating of reduced graphene oxide (RGO). The RGO-coated Na₃V₂O₂(PO₄)₂F cathode (hereafter denoted as NVOPF@RGO) delivers outstanding high-rate capability (93.6 mAh g^–1 at 60 C) and ultralong cycle stability (∼87% retention after 10 000 cycles at 50 C). This surface-enhanced material also exhibits excellent full-cell performance when coupled with the Fe_1–xS anode, which sustains a 94.3 mAh g^–1 specific capacity after 900 cycles at 20 C. The battery performance and stability of NVOPF@RGO are among the best in the state-of-the-art NASICON-based SIBs. Electrochemical measurements have shown that the coated RGO on NVOPF not only enhances its electric conductivity but also increases the apparent sodium ion diffusivity notably. We confirmed the structural reversibility and revealed the long/short-range structural evolutions of NVOPF@RGO upon electrochemical cycling by multinuclear solid-state nuclear magnetic resonance (ssNMR) combined with X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques.

中文翻译：

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揭示用于钠离子电池的高性能表面增强型NVOPF阴极材料的结构可逆性

聚阴离子型钠超离子导体（NASICON）Na ₃ V ₂（PO ₄）₂ F（NVOPF）是一种有前途的钠离子电池（SIB）阴极材料。但是，NVOPF在较高的速率下显示出相对较低的比容量和较差的长期性能。本文中，我们报告了通过引入还原性氧化石墨烯（RGO）的表面涂层，对NVOPF阴极材料的显着改进。涂有RGO的Na ₃ V ₂ O ₂（PO ₄）₂ F阴极（以下称为NVOPF @ RGO）具有出色的高倍率容量（93.6 mAh g ^–1在60°C时）和超长循环稳定性（在50°C下进行10 000次循环后保留率约87％）。与Fe _{1- x} S阳极结合使用时，这种表面增强材料还具有出色的全电池性能，该阳极可承受94.3 mAh g ^–1在20 C下900次循环后，电池的最大容量达到了最大容量。NVOPF @ RGO的电池性能和稳定性在基于NASICON的最新SIB中是最好的。电化学测量表明，在NVOPF上涂覆的RGO不仅增强了其电导率，而且显着提高了钠离子的表观扩散率。我们证实了结构的可逆性，并揭示了通过多核固态核磁共振（ssNMR）结合X射线衍射（XRD）和X射线光电子能谱（XPS）进行电化学循环后NVOPF @ RGO的长/短结构演变）技术。